There is a great need for devices or apparatuses which make it possible to identify or detect as regards their presence at a predetermined location objects which are provided with such devices or apparatuses in contactless manner and over a certain distance. An additional need exists to be able to change the resonant frequency with which such devices respond to such inquiries. An additional need exists to be able to tune the resonant circuit of the responder unit to match that of the interrogator unit, such as in the case the two units originally were matched in frequency and the resonant frequency of the responder unit has drifted or because of detuning due to environmental effects.
Heretofore, in this field, radio-frequency identification (RF-ID) transponders are permanently tuned to a resonant frequency at the factory. Certain environmental considerations such as presence of a para-magnetic or dia-magnetic material which lowers the inductance of an antenna and tunes it to a higher frequency (e.g. if the transponder is close to copper or aluminum). Other conditions might cause a change in the resonant frequency such as the ambient temperature or aging of the components. In the application of a small wireless transponder, it is desirable for any post-factory tuning to not require bulky electro-mechanical components as mechanically-variable capacitors or mechanically-variable inductors.